High pressure electric discharge tube



Feb. 7, 1967 J. A. c. JANSEN ETAL 3,303,377

HIGH PRESSURE ELECTRIC DISCHARGE TUBE Filed July 50, 1963 FIG.2

INVENTOR5 JOHANNES A.C. JANSEN TJ EPKE H. EKKEL BOOM United States Patent f 3,303,377 HIGH PRESSIHIE ELECTRIC DISCHARGE TUBE Johannes Adrianus Cornelis Jansen and Tjepke Hendrik Ekkelboom, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N .Y., a corporation of Delaware Filed July 30, 1963, Ser. No. 298,651 Claims priority, application Netherlands, Aug. 17, 1962,

. 282,235 1 Claim. (Cl. 313184) Our invention relates to a high-pressure electric discharge tube and more particularly to a compact source mercury vapor discharge lamp, i.e., a mercury vapor discharge lamp in which during operation the vapor pressure exceeds 25 atmospheres and in which the electrode distance is smaller than a fourth of the maximum diameter of the discharge space. The invention further relates to an electrode for use in such a discharge lamp.

The electrodes of mercury-vapor discharge lamps of the aforementioned kind usually comprise two cones the tips of which are arranged opposite each other, the arc being struck between the said tips during the operation of the lamp. Such lamps are disclosed, for example, in US. Patents 2,887,603 and 2,951,171.

When such a lamp is switched on, an arc is struck between the electrodes which generally does not start at the tips of the cones but at other places on the electrodes. The heating of the electrodes due to the passage of current results in a shortening of the discharge path until, when the operating temperature is attained, the arc is struck completely between the tips of the cone-shaped electrodes. A very small luminous surface then occurs which emits light of great intensity. Consequently, such lamps are very suitable for use in optical projection systems.

It is not necessary for the whole electrode to be conical; electrodes consisting of a conical part and a part connected thereto of, for example, cylindrical form also will be referred to hereinafter as conical electrodes.

In a mercury vapor discharge lamp of the kind described, the electrodes are thermally heavily loaded during operation. Therefore, materials for these electrodes are employed which can withstand very high temperatures. Consequently, the conical electrodes usually consist of tungsten (if a cylindrical part should be present, this may consist of another material, for example, molybdenum).

When a lamp comprising electrodes of pure tungsten is placed into operation, a very high voltage is required to ignite the arc, which results in a temporary but very strong disintegration of the tungsten. This disintegrated tungsten is deposited on the wall of the discharge space which usually consists of quartz glass and reduces, of course, the useful radiation of the tube. Moreover, a high ignition voltage is inconvenient since the apparatus required for producing this voltage is complicated and must be capable of withstanding high voltages.

In order to reduce the ignition voltage an ignition gas, for example, neon or argon or mixtures thereof has been tried. A considerable improvement is obtained, but this is still not suflicient. Consequently, it has been suggested to manufacture the electrodes from a mixture of tungsten and a material readily emitting electrons, in particular thorium oxide. With such electrodes, a low ignition voltage is indeed obtained; however, they suifer from the disadvantage in that during operation of the lamp, the arc burns very unstably. An are between two electrodes of pure tungsten burns much more stably. In order to obtain both a low ignition volt-age and a steadily burning arc, conical electrodes have been manufactured which are provided with a cavity in which material readily emitting electrons, in particular thorium oxide is disposed. From 3,303,377 Patented Feb. 7, 1967 this cavity, one or more bores extend towards the surface of the electrode. Emitting material may emerge at this surface through these bores and may contribute there to a low ignition voltage. In this structure, the tip of the conical electrode consists of pure tungsten and so the arc can burn steadily at this tip during operation. It has been found, however, that this structure has the disadvantage that an excess of emitting material readily emerges from the supply chamber through the bores at the surface of the electrode, evaporates there and finally forms a grey precipitate on the wall of the discharge space, as a result of which the emitted light is scattered, which is contrary to the use of the lamp.

A principal object of our invention is to provide a high-pressure mercury discharge lamp which employs tungsten electrodes but which is provided with an improved igniter for initiating the discahrge. According to theinvention, our lamp comprises two conical tungsten electrodes arranged opposite each other, one electrode being provided at a distance from the tip of the cone which is smaller than four times and larger than twice the distance between the electrodes, with a bore which is filled with a thoriated tungsten wire.

The bore, in which the tungsten wire is disposed, need not be at right angles to the conical surface; the distance from the tip to the thoriated tungsten wire is measured along the conical surface to the axis of the bore.

By choosing the distance between the aforementioned limits, the are which starts at the thoriated wire readily leaps to the tip of the cone. At the same time, the danger is reduced that a gray precipitate is formed on the wall of the discharge space during operation, since only a small amount of thorium oxide is present in the wire and the arc does not continue at the thoriated wire during operation of the lamp.

The diameter of the bore in which the thoriated tungsten wire is disposed is preferably chosen smaller than one-fifth the maximum diameter of the conical tungsten electrode. Rapid heating of this thoriated wire is thus ensured, as a result of which the ignition time is shortened.

Preferably, the thoriated tungsten wire projects slightly, but by less than of the maximum cone diameter, from the conical surface of the electrode. The arc is thus struck more readily and always at the same place, i.e. on the projecting part.

The invention will now be described more fully with reference to the accompanying drawing in which:

FIG. 1 shows a discharge lamp in accordance with the invention, and

FIG. 2 shows enlarged and drawn to a scale 1:10 the electrodes used in the tube shown in FIG. 1.

Referring now to FIG. 1, 1 is a bulb of quartz glass in which the electrodes 2 and 3 are provided, the ends of which facing each other are conical. According to the invention, the electrode 3 contains a tungsten wire provided in a bore in the conical end of this electrode. This appears more clearly from FIG. 2.

The electrode 3 has a conical end 4 in which a bore 5 is provided. In this bore 5, a thoriated tungsten wire is provided which projects slightly from the conical surface. According to the above condition, the distance b is smaller than four times the distance a and larger than twice this distance. In the electrode 3, provision is made of grooves 6 in order to reduce the dissipation of heat, which is known per se.

While we have described our invention with reference to a specific embodiment and applications thereof, other modifications will be apparent to those skilled in this art without departing from the spirit and scope thereof as defined in the appended claim.

What we claim is:

A high pressure electric discharge tube comprising a 3 7 4 light-transmissible envelope, a gas filling said envelope at References Cited by the Examiner a pressure of several atmospheres, a pair of axially aligned UN STATES PATENTS spaced conlcal tungsten electrodes Wlthln said envelope, the spacing of the electrodes being small compared with 2,087,735 6/1937 Plram 313" 184 the diameter of the envelope, and at least one of said elec- 5 2,194,652 3/1940 Fmedman- 2,459,579 1/1949 Noel 313184 trodes having a bore therein spaced from the apex of the electrode a distance which is smaller than four times and I FOREIGN PATENTS larger than twice the distance between the electrodes,

and a thoriated tungsten wire filling said bore, said wire 677,551 8/1952 Great Bummprojecting from the surface of said electrode a distance 10 not exceeding of the maximum diameter of said DAVID y Elm/"mgrconical electrode. 

